Page 178 - Reservoir Geomechanics

161 Faults and fractures at depth

illustrated that extensional normal faulting was occurring along mid-ocean spreading
centers and the appropriate sense of lateral slip occurred on transform faults (see the
review by Stein and Klosko 2002).
With respect to the orientation of in situ stress, the advantages of utilizing well-
constrained earthquake focal plane mechanisms to map the stress field are fairly
obvious: earthquakes record stress-induced deformation at mid-crustal depths, they
sample relatively large volumes of rock and, due to the continued improvement of
regional and global networks, more well-constrained focal mechanisms for mapping
the stress field are available now than ever before. However, it is important to keep
in mind that focal plane mechanisms record deformation and not stress. The P-
and T-axes shown in Figure 5.11 are, by definition, the bisectors of the dilatational
and compressional quadrants of the focal mechanism. Thus, they are not the maxi-
mum and minimum principal stress directions (as is sometimes assumed) but are the
compressional and extensional strain directions for slip on either of the two possible
faults. As most crustal earthquakes appear to occur on pre-existing faults (rather than
resulting from new fault breaks), the slip vector is a function both of the orientation of
the fault and the orientation and relative magnitude of the principal stresses, and the P-
and T-axes of the focal plane mechanism do not correlate directly with principal stress
directions. In an attempt to rectify this problem, Raleigh, Healy et al.(1972) showed
that if the nodal plane of the focal mechanism corresponding to the fault is known, it
is preferable not to use the P-axes of the focal-plane mechanism but instead to assume
an angle between the maximum horizontal stress and the fault plane defined by the
coefficient of friction of the rock. Because the coefficient of friction of many rocks is
often about 0.6, Raleigh, Healy et al.(1972) suggested that the expected angle between
the fault plane and the direction of maximum principal stress would be expected to be
◦
about 30 . Unfortunately, for intraplate earthquakes (those of most interest here), we
usually do not know which focal plane corresponds to the fault plane. Nevertheless, in
most intraplate areas, P-axes from well-constrained focal plane mechanisms do seem
to represent a reasonable approximation of the maximum horizontal stress direction,
apparently because intraplate earthquakes do not seem to occur on faults with extremely
low friction (Zoback and Zoback 1980, 1989; Zoback, Zoback et al. 1989) and give
an indication of relative stress magnitude (normal, strike-slip or reverse faulting). This
will be discussed in more detail in Chapter 9.
As mentioned in Chapter 1,if the coefficient of friction of the fault is quite low,
the direction of maximum compression can be anywhere in the dilatational quadrant
and the P-axis can differ from the true maximum stress direction by as much as 45 ◦
(MacKenzie 1969). In fact, studies such as Zoback, Zoback et al.(1987)excluded as
tectonic stress indicators right-lateral strike-slip focal plane mechanisms right on the
San Andreas fault as did subsequent stress compilations at global scale as discussed
in Chapter 9.In the case of the San Andreas, appreciable heat flow data collected
in the vicinity of the San Andreas show no evidence of frictionally generated heat